High efficiency Iridium (III) bis (4-phenylthieno [3,2-c] pyridinato-N,C-2') acetylacetonate (PO-01) based yellow organic light-emitting devices are fabricated by employing multiple emission layers. The efficiency of the device using 4,4',4 ''-tris(N-carbazolyl) triphenylamine (TCTA) as potential barrier layer (PBL) outperforms those devices based on other PBLs and detailed analysis is carried out to reveal the mechanisms. A forward-viewing current efficiency (CE) of 65.21 cd/A, which corresponds to a maximum total CE of 110.85 cd/A is achieved at 335.8 cd/m(2) in the optimized device without any outcoupling enhancement structures. (C) 2015 Elsevier Ltd. All rights reserved.